Removing salts from polyacrylonitrile



Patented Aug. 11, 1953 REMOVING SALTS FROM POLY- ACRYLONITRILE George W.Stanton, Walnut Creek, and Theodore B. Lefierdink, Concord, Calif.,assignors to The Dow Chemical Company, Midland, Mich.,- a corporation ofDelaware N Drawing. Application May 28,1951, Serial N0. 228,759

3 Claims.

This invention relates to a method for removing metal salts frompolymeric films or filaments composed chiefly of acrylonitrile. It isconcerned particularly with such a method for minimizing the saltcontent of fibers of polyacrylonitrile which have been spun from aqueoussaline solutions of the polymer.

It is taught by Rein in U. S. 2,140,921 and by Kropa in U. S. 2,356,767that polyacrylonitrile is soluble in concentrated aqueous solutions ofcertain highly hydrated metal salts, such as zinc chloride, and thatsuch solutions can be spun to make fibers. Such fibers have beencriticised in numerous patents as being weak, brittle and full of voids.Such brittleness or weakness may be due in some cases to the voidsresulting from too rapid extraction of the salt during coagulation ofthe filamentary extruded product, but in others it is due to thepresence of some of the salt in the final spun product.

In a group of applications filed concurrently herewith by one or both ofthe present inventors and others, Serial Nos. 228,751 to 228,758inclusive, it is disclosed that improved spun products can be made frompolyacrylonitrile when the latter is first dissolved in an aqueoussolution of at least 30 per cent, and usually not over 50 per cent, of asolvent salt and a lesser amount, from 5 to 25 per cent of a non-solventsalt, in proportions such that the aggregate amount of the two salts isat least 55 per cent, by weight. The solvent salts are those whoseanions are at least as high in the following table as their cations, andthe non-solvent salts are those whose anions are lower in the table thantheir cations. To be useful, both types of salts employed must bemutually soluble in water.

Anions in Decreasing Order of Efiectlveness Cations in Increasing Orderof Effectiveness SCN" While fibers spun from solutions in such mixedsalts are superior to those made from solutions in single solvent salts,it is desirable for most purposes that the amount of salt or salts leftin the finished fiber be reduced to a minimum. It has been found thatthe rate of extraction of the last few percent ofsalts from the fibersby water is 'very slow, and often may be considered impractical for usein a continuous fiber spinning process.

It is a principal object of the present invention to provide a practicaland rapid method for reducing the salt content of articles such as filmsor filaments which have been formed from aqueous saline solutions of thepolymer of acrylonitrile or of copolymers of at least per centacrylonitrile with another monoethylenically unsaturated compond.

In accordance with the present invention, which will be described withreference to polyacrylonitrile fibers, the desired salt removal iseffected after the fibers have been coagulated and stretched in thedesired manner. The fiber forming process may entail coagulation of thepolymer solution as it is extruded into water, acid or dilute brine,followed by cold stretching or hot stretching, or both, with followingor in tervening treatments of the fiber with water or other coagulant toreduce the salt content to a value usually between 2 and 5 per cent.Such an amount of residual salt is not detrimental during a subsequentdrying operation, though the presence of over 5 per cent salt usuallycauses the fibers to fuse when heated to C. or so during drying. A muchlower salt content is required when the fibers are to be used in makingwoven or knit goods which may be worn in contact with sensitive skins.Hence after the final wet stretching treatment, and before the fibersare dried, they are conducted through an aqueous solution of an agentcapable of forming a complex with the metal ion or ions of the salt orsalts in the fibers. Such agents include chelating agents and othermetal sequestering agents. The preferred agents include the alkali metalsalts of ethylene diamine tetraacetic acid and the alkali metal salts ofthe polyphosphoric acids. These are capable of forming complexes with,and of removing rapidly from solution, the commoner polyvalent metalions which may be present in the fibers, including those of zinc,calcium, magnesium, tin, aluminum, antimony, arsenic, bismuth, andothers. The treatment is carried out while the fibers are still wet froma prior step in the process, as it has been found that dried fiberscontaining salt lose it very slowly to a solution of one of thecomplex-forming or sequestering agents, as they do to plain water.

The following example illustrates the mac-- tice of the invention, butis not to be construed as limiting:

A solution was prepared consisting of 7 per cent by weight ofpolyacrylonitrile, having a Staudinger molecular weight of 76,100, 31.6per cent zinc chloride, 23.2 per cent calcium chloride and 38.2 per centwater. This solution, having a viscosity of 300 poises,-was extrudedthrough a 100-hole spinneret, having openings 0.0025 inch in diameter,into Water at C. The socoagulated tow of filaments was cold-stretched inair 1.8 times its initial length, and was hotstretched in wet steam 2.4times its cold-stretched length. The tow of stretched fibers was washedfor 10 seconds with water. Analysis of the product at various stages inthe process to this point showed the following values:

One portion of the tow was dried to l per cent moisture content, and had2.9 per cent zinc chloride and 1.7 per cent calcium chloride. Anotherportion of the wet tow was soaked in water for one minute, and, whendried, had 0.35 per cent zinc chloride and 0.22 per cent calciumchloride. When, however, the wet tow was led through a 10 per centsolution in water at pH 8 of the tetrasodium salt of ethylene diaminetetracetic acid for one minute, the subsequently dried fibers had 0.04per cent zinc chloride and 0.08 per cent calcium chloride. By way ofcontrast, if the tow was dried before being exposed to the solution ofsequestering or chelating agent, soaking in such a solution for 18 hoursstill left over 1 per cent total salt in the redried fibers.

In other tests, when the coagulated and stretched product was immersedin aqueous solutions of metal complexing agents while the product wasstill wet from a prior step in the process, the alkali metal salts ofethylene diamine tetraacetic acid and of polyphosphoric acids removedthe polyvalent metal salts effectively from the product. Thus when thetreated product has been digested with nitric acid, burned to constantweight, and the ash analyzed photometrically for calcium andcolorimetrically with dithiazine for zinc, values as low as nil for bothelements have been obtained, and total values for the two are seldomfound to exceed 0.2 per cent. The complexing agents have been found tobe most effective when their solutions are adjusted to a pH value near8, as their normally alkaline condition, pH 11 or higher, tends todiscolor the fibers.

The invention has been illustrated with respect to the treatment offibers produced by wet spinning of aqueous saline solutions ofpolyacrylonitrile. It is applicable as well to the removal of salts fromsuch other products as films made by extrusion and coagulation of suchsaline solutions.

We claim:

1. The method as claimed in claim 3, wherein the solution of said alkalimetal salt is employed at a pH of about 8.

2. The method as claimed in claim 3, wherein the shaped article is awet-spun fiber of polyacrylonitrile.

3. In a method for making shaped articles by coagulation of an aqueouspolyvalent metal saltcontaining saline solution of a polymer containingat least per cent acrylonitrile in the polymer molecule, the improvementwhich consists in immersing the article, before it has been dried, in anaqueous solution of a water-soluble alkali metal salt of ethylenediamine tetraacetic acid as a sequestering agent for the polyvalentmetal ions in the salt contained in the coagulated article, until thesalt content is reduced to a negligible value, and then drying thearticle.

Martell et al., Properties and Uses of Ethylenediamine Tetra Acetic Acidand Its Salts, Beersworth Chemical Co. 194.8Frarningham, Mass, page '7.

Number

3. IN A METHOD FOR MAKING SHAPED ARTICLES BY COAGULATION OF AN AQUEOUSPOLYVALENT METAL SALTCONTAINING SALINE SOLUTION OF A POLYMER CONTAININGAT LEAST 85 PER CENT ACRYLONITRILE IN THE POLYMER MOLECULE, THEIMPROVEMENT WHICH CONSISTS IN IMMERSING THE ARTICLE, BEFORE IT HAS BEENDRIED, IN AN AQUEOUS SOLUTION OF A WATER-SOLUBLE ALKALI METAL SALT OFETHYLENE DIAMINE TETRAACETIC ACID AS A SEQUESTERING AGENT FOR THEPOLYVALENT METAL IONS IN THE SALT CONTAINED IN THE COAGULATED ARTICLE,UNTIL THE SALT CONTENT IS REDUCED TO A NEGLIGIBLE VALUE, AND THEN DRYINGTHE ARTICLE.